Surface electronic structure of [XMIm]Cl probed by surface-sensitive spectroscopy.

Area of Science:

• Surface science
• Physical chemistry
• Materials science

Background:

• Ionic liquids (ILs) are tunable solvents with diverse applications.
• Understanding the surface composition and molecular orientation of ILs is crucial for their performance.
• Electron spectroscopy offers insights into the electronic structure and surface properties of materials.

Purpose of the Study:

• To investigate the density of states (DOS) of the surface and near-surface regions of [XMIm]Cl ionic liquids.
• To determine the molecular and ionic arrangement and orientation at the IL surface.
• To correlate surface composition with varying alkyl chain lengths (X=octyl, hexyl, butyl, ethyl).

Main Methods:

• Utilized electron spectroscopy techniques with varying surface sensitivities: Metastable Induced Electron Spectroscopy (MIES), Ultraviolet Photoelectron Spectroscopy (UPS), and X-ray Photoelectron Spectroscopy (XPS).
• Employed Density Functional Theory (DFT) calculations for spectral assignment and reconstruction.
• Performed angular-resolved XPS to probe surface depth profiles.

Main Results:

• Angular-resolved XPS confirmed the dominance of alkyl chains at the outermost surface.
• UPS spectra showed a decreasing chlorine signal with increasing alkyl chain length.
• MIES data for [OMIm]Cl revealed no chlorine-induced features, with spectra dominated by the alkyl chain, indicating a distinct surface sensitivity compared to UPS and XPS.

Conclusions:

• The surface composition of [XMIm]Cl ionic liquids is strongly influenced by alkyl chain length.
• Longer alkyl chains effectively shield the ionic liquid surface, reducing the presence of anions like chloride.
• MIES provides unique information about the outermost molecular layer, distinct from UPS and XPS.